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Archives of Applied Science Research, 2015, 7 (1):42-47 (http://scholarsresearchlibrary.com/archive.html)

ISSN 0975-508X CODEN (USA) AASRC9

Na ₂ Ti ₆ O ₁₃ belts: A comparative study on structural, optical and

morphological properties prepared by sol-gel and solid state reaction routes

Shamim Ahamad Khan ^a* , Irfan Ali Khan ^a , Mohd. Sajid Khan ^b , Manaal Zahera ^b and Mohd. Arshad ^c

a

Department of Applied Physics (Nanoscience), Integral University, Kursi Road, Lucknow, India

b

Department of Biosciences, Integral University, Kursi Road, Lucknow, India

c

Centre of Excellence in Material Science (Nanomaterials), Department of Applied Physics, Z.H. College of Engineering and Technology, AMU, Aligarh, India

_____________________________________________________________________________________________

ABSTRACT

In this paper sodium hexatitanate (Na

Ti

O

) belts were prepared by two specific synthesis process such as sol-gel and solid state reaction methods. The structural, morphological and optical characteristics were performed by XRD, SEM and UV-vis absorption spectroscopy. The crystallite size and unit cell volume of sodium hexatitanates (Na

Ti

O

) synthesized by sol-gel method have less than same obtained by solid state reaction method. The energy band gap (3.18eV) of Na

Ti

O

prepared by sol-gel method was obtained larger than (3.07eV), prepared by solid state reaction method. Beyond it, Na

Ti

O

was tested as photocatalyst on the degradation of methyl orange under visible light and obtained better activity for the sample synthesized by sol-gel reaction route. From the observed data we have concluded that the sodium hexatitanate (Na

Ti

O

) whiskers synthesized by sol-gel method showed superior properties than the solid state reaction method.

Keywords: Sodium hexatitanate (Na

Ti

O

), Sol-gel method, Solid state reaction route, Energy band gap, Photocatalyst XRD, SEM.

_____________________________________________________________________________________________

INTRODUCTION

In recent years, alkalititanates are the derivative of titania or titanium dioxide (TiO

) which is a newly synthesized compound in chemical world. The general formula of alkali titanate is A

Ti

O

(where A = Li, Na and K etc and n

= 2-8) [1-2]. Alkali titanates are well known functional materials because of their outstanding electronic, magnetic,

optical, catalytic, mechanical properties and thermal stability. Among all, titanates are of great interest in many

important applications such as ion exchange, photoluminescence[3], photochemical catalyst, RuO

-supporting

Na

Ti

O

show high photocatalytic activities for water decomposition [4-6], filter materials, reinforced materials,

acoustic absorbing materials and as well as heat insulator[7]. It has also been reported that Na

Ti

O

have various

interesting technological applications such as ceramic capacitors, dielectric sensors, biosensors [8-9]. These

properties and applications are related to the morphology, size and crystallinity of alkalititanates. In order to

improve the functional properties such as photocatalytic activities or ion exchange ability, it is highly desirable to

control the size and morphology of the titanates[10]. Alkali titanates are comparatively cheap fibrous materials

which have high thermal durability, chemical resistivity and dispersibility. All these properties make them useful as

reinforced materials, heat insulating paints, automotive brake linings and photocatalytic materials [5, 11-19]

43

Sodium hexatitanates could be synthesized by sol-gel method, molten salt calcinations routes, novel hydrothermal process and solid state reaction methods in molar ratio. In this study nano sodium hexatitanates (Na

Ti

O

) were successfully prepared by sol-gel and solid state reaction methods. Besides these properties the photocatalytic degradation of methyl orange under visible light at room temperature was also tested for both the samples. The comparative studies were done on structural, optical and morphological behavior of Na

Ti

O

synthesized by two different methods.

MATERIALS AND METHODS

2.1 Synthesis-

The compound Na

Ti

O

was prepared by sol–gel method using titanium (IV) isopropoxide, sodium acetate, ethanol and distilled water. In this method, an adequate stoichiometric amount of titanium isopropoxide was placed into a 500 ml round bottom flask. Sodium acetate was dissolved in 20 ml of distilled water and ethanol, and then added drop by drop into the flask containing the titanium isopropoxide. The mixture was heated under reflux at 70

0

C during 72 h under constant stirring. Basic pH of the solution was set and controlled by using small amount of NH

OH. After heating, the solution was aged at 25

C for 24 h until the gel formation. The gel was heat treated at different temperature (T= 1000

C) for 8 h in an electric furnace.

In typical solid state reaction method sodium hexatitanate belts were synthesized by mixing adequate stoichiometric amounts of Na

CO

and TiO

powder with acetone in mortar and grinded for 1 hour. After mixing, the mixture was placed in an alumina crucible and calcinated at 1000

c for 12 h in an electric furnace.

2.2 Characterization- (i) XRD-

Structural properties of sodium hexatitanates belts (Na

Ti

O

) synthesized by sol-gel and solid state reaction methods were determined by X-ray diffraction (XRD) using Rigaku-miniflex. X-ray diffractometer with Cu-Kα radiations (λ = 0.15406 nm) in 2θ range from 10

to 70

with scan rate at 2

/min at room temperature. Lattice parameters were calculated by applying the equation:

= + + −

(1)

(ii) UV-vis absorption Spectroscopy-

UV-vis absorption spectra for the samples were taken by Perkin Elmer Lambda 35 UV-visible spectrometer. The energy band gap was calculated by using Tauc relationship:

ℎ = (ℎ − ) (2)

For alkali titanates nanoparticles the absorbance coefficient α obey the above relation with = (for direct allowed band gap semiconductor). The photocatalytic tests were carried out by using aqueous solution of methyl orange (1.0 X 10

M) under irradiation of visible light. 0.02 gm Na

Ti

O

powders (synthesized by sol-gel and solid state methods) were weighted and used as photocatalysts. The photocatalyst and methyl orange suspension was kept under the irradiation of visible light at room temperature with continuous stirring. The characteristic absorption of methyl orange λ = 464 nm was chosen to observe the photocatalytic degradation process on UV–visible spectrometer by the exposure visible light with increase in irradiation time.

(iii) SEM-

Morphologies and microstructures of sodium hexatitanates belts (Na

Ti

O

) were examined using scanning electron microscope (SEM, VEGA II LSU), operating at 30 kV.

RESULTS AND DISCUSSION

3.1 Structural Properties-

Fig. 1 shows the XRD patterns of sodium hexatitanate belts (Na

Ti

O

) prepared by sol-gel and solid state reaction

routes recorded at room temperature. The peak position of each sample exhibits the monoclinic structure and no

other impurity peak was observed in XRD pattern which confirms the single phase formation. The lattice constants

were calculated from XRD pattern by using equation 1 and shown in table 1. The unit cell parameters also reported by S. Andersson et al [20].

Table 1 Lattice parameters and cell volume of Na

Ti

O

Sample Lattice parameter a (Å)

Lattice parameter b (Å)

Lattice parameter

c (Å) β

Unit cell volume (Å)

Na

Ti

O

(Sol-gel) 15.168 3.720 9.122 99.246 508.021

Na

Ti

O

(Solid state) 15.136 3.752 9.163 99.302 513.526

From table 1, it can be observed that sol-gel method shrink the unit cell volume as compare to solid state reaction route; this is because of small crystallite size in sol-gel formation.

Fig. 1 XRD pattern of sodium hexatitanates (Na

Ti

O

) synthesized by sol-gel and solid state

Fig. 2: Low magnification SEM image of Na

Ti

O

prepared by sol-gel (a) and solid state reaction method (b) respectively

(a) (b)

45

3.2 Morphological Properties-

The morphology of the sodium hexatitanates (Na

Ti

O

) prepared by sol–gel method and solid state reaction route were characterized by scanning electron microscope (SEM) shown in Fig. 2. The fig. 2(a) represents the low magnification image of sodium hexatitanates synthesized by sol-gel showing nanobelts with cross sectional width less than 100 nm while fig. 2 (b) shows the low magnification of the same particles, have larger thickness of belts synthesized by solid state reaction method. This attributed that by using sol-gel method we can control the synthesis of particles with a particular shape and size.

3.3 Optical Properties-

UV-vis absorption spectrum of Na

Ti

O

of nanobelts in order to obtain clues regarding the electronic structure, shown in fig. 3. The absorbance is expected to depend on several factors, such as band gap, surface roughness and impurity centers. The absorption spectrum obtained by sol-gel sample was shifted towards the shorter wavelength region. This effect might be due to the quantum size effect [21]. In order to calculate direct band gap Tauc relationship used [22].

ℎ = (ℎ − )

Where A is a constant that depends on the properties of material, α is the absorption coefficient given by α = 2.303 (Ab/t), where, Ab is absorbance and t is the cuvette thickness (1 cm in present case), hυ is the photonic energy, Eg is the energy band gap. n has different values depends on electronic transition types, for a allowed direct transition n =

½, a forbidden direct transition n = 3/2, a allowed indirect transition n = 2 and for a forbidden indirect transition n = 3 [23-24]. The plots of (αhυ)

versus hυ for Na

Ti

O

samples are shown in Fig.3 (a) and Fig. 3 (b).

The Tauc relationship was used to calculate the direct allowed band gap for the samples of sodium hexatitanates (Na

Ti

O

) nanobelts synthesized by sol-gel and solid state reaction methods. The measured band gap was found to be 3.18 eV for sodium hexatitanates synthesized by sol-gel, which is higher than the value 3.07 eV found for the same, synthesized by solid state reaction method. This can be incriminated due to the quantum confinement effect of nanoparticles [25]. This result agrees with the previously reported [26].

3.4 Photocatalytic Test-

In this experiment photocatalytic activity of prepared sodium hexatitanate (Na

Ti

O

) by two different methods

were tested by the degradation of methyl orange under visible light irradiation. It was evaluated by measuring the

photodegradation of a solution of methyl orange (1.0 x 10

M, 400 ml in a quartz vessel) in the presence of 0.02 gm

Na

Ti

O

powder which was exposured to visible light at room temperature. Fig. 4 shows the absorbance decay

with time 0 to 1 hour regarding the progress of the reaction with stirring the suspension. The color of the suspension

faded simultaneously with an increase in irradiation time. Lastly the suspension of methyl orange changed from

orange color to approximately colorless. Hence, the absorption peak value at λ = 464 nm decreases sharply as the

visible light exposure time increases. The color of methyl orange solution remains unchanged when no Na

Ti

O

powder were added in visible light irradiation. Thus sodium hexatitanate (Na

Ti

O

) act as affective photocatalytic

material and degrade the methyl orange molecule into several different smaller molecules. In this work sodium

hexatitanate (Na

Ti

O

) with a rectangular tunnel structure has high efficiency to produce photoexcited charges

(electron and holes) [6], which are thought to be associated with its high photocatalytic activity. Accordingly it has

been observed from Fig. 4. that the phtotocatalytic ability were found to be better for the Na

Ti

O

prepared by sol-

gel method as compare to the same, prepared by solid state reaction route because the larger surface area or smaller

particle size obtained in sol-gel method of preparation.

Fig. 3 : UV-vis absorption spectrum for sodium hexatitanates (Na

Ti

O

). (a & b): Plots of (αhυ)

versus hυ for both the specimen.

Fig. 4: Absorbance spectra of methyl orange solution (1.0 x 10

M, 400 ml) in the presence of 0.02 gm of Na

Ti

O

nanobelts prepared by (a) solid state and (b) sol-gel reaction routes under the exposure of visible light

(3a) (3b)

(4a)

(4b)

47

CONCLUSION

Sodium hexatitanate (Na

Ti

O

) has been prepared successfully as a single phase by sol-gel and as well as by solid state reaction routes. Its crystal structure coincides to a monoclinic as confirmed by XRD pattern. Surface morphology was studied by scanning electron microscope (SEM) of single phase and exposed as nanobelts. The optical investigations have been carried out using optical absorbance. The optical band gap of the samples were measured from the Tauc relationship and observed that energy band gap increases with the decrease in particle size.

Na

Ti

O

was tested as photocatalyst on the degradation of methyl orange under visible light and better results were found for the sample prepared by sol-gel than the same prepared by solid state reaction routes.

Acknowledgements

Authors are thankful to Integral University and Centre of Excellence in Material Science (Nanomaterials), AMU, Aligarh for providing infrastructure for the experiments carried out for the research.

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